In the graphic arts industries and the like, a light-sensitive material to make an original transparency signifies light-sensitive materials which are used for converting continuous-tone images of originals into halftone images or for photographing line originals or the like, which is involved in a photomechanical process.
In the usual production of printing plates utilizing such light-sensitive materials are described above, partial or overall minute retouch of image is carried out. This is done with the intention of reproducing the delicate tones of originals so that they have excellent printing characteristics or unsatisfying the artistic expression of the printed image. This is frequently carried out by subjecting these materials to a processing referred to as a reduction processing. This involves reducing the dot area of halftone images, or increasing or decreasing the width of line images, or so on.
Accordingly, whether or not the light-sensitive material has an aptitude for a reduction processing has become very important of such materials.
To subject a light-sensitive material for photomechanical process having a halftone image or a line image formed thereon through an exposure and development process, a method is used which comprises contacting metallic silver forming the halftone or line image with a reducer. Many kinds of reducers are known. For example, Mees, The Theory of the Photographic Process, pages 738-739 (1954, Macmillan Co.) describes reducers containing permanganates, ferric salts, ceric salts, potassium ferricyanide, persulfates and bichromates as reducing components.
Since the reducing treatment is after all the oxidation and dissolving of a silver image, when the dot area of a halftone image is decreased by the reducing treatment, the blackening density of the dots is simultaneously decreased. Accordingly, the range modifiable by the reducing treatment is restricted by the degree of decrease of the blackening density per dot which occurs during the decreasing of the dot area. In other words, the measure of the modifiable range of a halftone image can be expressed by the extent of the decrease of the dot area which can be effected while the blackening density per dot is maintained above a certain fixed value.
The extent to which the dot area has been decreased from that before treatment is determined when as a result of the reducing treatment, the blackening density of a dot is decreased to a minimum limit required in the photomechanical process, and this is expressed in the present specification as the "reduction extent". The larger the reduction extent, the higher is the adaptability of the light-sensitive material for photomechanical process to the reducing treatment.
As a technique for increasing adaptability to a reducing treatment, a method involving using a reducer containing a mercapto compound during the reducing treatment is known, as described, for example, in Japanese Patent Application (OPI) No. 68419/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). The reducer thereof is special and difficult to use, because the rate of reduction differs from those of reducers generally used. If an emulsion film is rendered soft and the covering power and the density are increased, it is possible to increase the reduction extent and improve the reducing treatment characteristics. However, this method cannot provide the generally required film strength.
From a technical viewpoint, the most effective method among the techniques of increasing the reduction extent and improving the reducing treatment adaptability that have been known is to increase the amount of silver forming an image. This is because the modifiable range of the image by the reduction treatment becomes broader as the amount of silver forming the silver image per unit area is larger. Accordingly, the reduction extent can be broadened by increasing the amount of silver halide used in a light-sensitive material for photomechanical process per unit area. However, as is well known, silver is very expensive and valuable, thus increasing the amount of silver coated is undesirable in view of the cost of the resulting light-sensitive material for photomechanical process.
It is, therefore, one of the important problems in the art to produce light-sensitive materials for photomechanical process having desirable reducing characteristics as noted above while using as little silver as possible.
Japanese Patent Application (OPI) No. 42039/83, by the present applicant, has proposed that in order to solve the above problem, a light-sensitive upper layer having a longer melting time than the melting time of a silver halide emulsion layer be formed on the silver halide emulsion layer. By this means, the reducing treatment adaptability of the resulting light-sensitive material can be greatly improved without increasing the amount of silver coated. However, in order to increase the melting time of the light-insensitive upper layer, it must be hardened independently of the emulsion layer. Hence, it has been found that the coating of this layer is not sufficiently easy and care must be taken, or reticulation occurs.